[dev.cmdgo] cmd/go: thread through modroots providing replacements

modload.Replacement and modload.resolveReplacement now also return
the modroot of the module providing a replacement so that we can
correctly construct the path of a replaced module (because the
path in the module.Version is relative to the modroot).

For #45713
Change-Id: I8c69ccbcc1f40201071e35fcf93d6b5d0ed4cdf7
Reviewed-on: https://go-review.googlesource.com/c/go/+/334941


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>